60 INSECT PHYSIOLOGY 



and hypoxanthine may occur; and many insects will break down 

 their uric acid, wholly or in part, to allantoine or allantoic acid. 

 Allantoine is plentiful in the excreta of blowfly maggots. The enzyme 

 uricase which converts uric acid to allantoine is particularly active in 

 many Heteroptera (e.g. Dysdercus) which excrete most of their 

 nitrogen in this form. 



The reaction of the urine naturally varies with the diet, and the 

 period after feeding; the thick meconium of Lepidoptera has a/?H 

 of 5*8-6-3 ; the similar urine of the bug Rhodnius prolixus shows the 

 same kind of range. 



When uric acid is present in the urine it is usually in the form of 

 crystalline spheres with a radial striation. These spheres are com- 

 monly stated to consist of ammonium acid urate, or of sodium or 

 potassium acid urate; but in Rhodnius, almost the only insect in 

 which this question has been investigated chemically, they consist 

 mainly of free uric acid; and it is possible that this is also the case in 

 other insects. 



Besides these crystals of uric acid, the Malpighian tubes may con- 

 tain solid spheres or granules of calcium or magnesium carbonate 

 and phosphate and, notably in the larvae of Lepidoptera, crystals of 

 calcium oxalate. The oxalate is possibly derived from the preformed 

 oxalic acid in the food, but the significance of the accumulated 

 carbonates is uncertain. They are sometimes used by the pupating 

 larva to strenghten its cocoon (Cerambycidae, Col.) or its puparium 

 (Agromyzidae, Dipt.) or by the female to calcify the shells of her 

 eggs (Phasmidae, Orth.), but these are probably secondary adapta- 

 tions. It has been suggested that the carbonates may constitute a 

 method of eliminating carbon dioxide; but such a mechanism seems 

 uncalled for, and in any case the amount of carbon dioxide bound in 

 this way must form but a fraction of what the insect produces. Per- 

 haps these carbonates simply provide a mechanism for getting rid of 

 excess alkali; it is at least noteworthy that they occur chiefly in 

 saprophagous and phytophagous larvae of Diptera (Stratiomyiidae, 

 Drosophilidae, Agromyzidae, and many more) whose food might be 

 expected to contain an excess of fixed base. Their urine must there- 

 fore be alkaline, and in the presence of the carbon dioxide of the 

 tissues, if calcium or magneisum ions are present in quantity, 



